In printing by a printer using an ink-jet printhead (to be referred to as a printhead hereinafter) having a plurality of nozzles, if the printhead includes even one ink discharge failure nozzle (to be referred to as a discharge failure nozzle hereinafter), a white stripe appears on a printed product, and the printed product cannot be used duly. When even one discharge failure nozzle exists in the printhead and the discharge failure occurs due to a cause which cannot be solved even by a recovery process, there is no means for solving this printing failure except that the printhead having the discharge failure nozzle is not used.
More specifically, when an unrecoverable discharge failure nozzle is detected during printhead manufacturing process, the printhead having the discharge failure nozzle must be discarded. If a discharge failure nozzle unrecoverable by the recovery process is detected in the printhead after the printhead is passed to the user, the user must exchange the printhead.
Not only a discharge failure nozzle, but also a nozzle which cannot correctly print due to a discharge direction greatly deviated from a normal direction, and a nozzle which negatively influences printing because the size of a discharged ink droplet is greatly different from a normal one are not suitable for normal printing. These nozzles are treated as abnormal nozzles, similar to the discharge failure nozzle. A printhead having such abnormal nozzle is regarded as a defective printhead.
Occurrence of a discharge failure nozzle (to be also referred to as an abnormal nozzle hereinafter) in the printhead imposes an economic burden on both the manufacturer and user.
Recent printheads are equipped with a large number of ink discharge nozzles (to be referred to as nozzles hereinafter). For example, assuming that the printhead has 512 nozzles in order to discharge an ink of one color, a printhead designed to discharge inks of six different colors has a total of 3,072 nozzles. As the number of nozzles increases, the possibility at which discharge failure nozzles occur increases. Demands have arisen for a measure against a discharge failure nozzle so as to reduce the economic burden on both the manufacturer and user.
In order to avoid this situation, several printer manufacturers have recently proposed a technique associated with so-called discharge failure complement of complementing printing of a discharge failure nozzle in the printhead. For example, Japanese Patent Publication Laid Open No. 6-226982 discloses a technique in which when a discharge failure nozzle exists in the printhead, printing data corresponding to the position of the discharge failure nozzle is printed by a normal nozzle.
Assume that complementary printing is performed by, e.g., multi-pass printing with a normal nozzle using printing data corresponding to the position of a discharge failure nozzle. In multi-pass printing, a printing medium is generally conveyed by a width obtained by dividing the printing width of the printhead by the number of passes of multi-pass printing every time printing by one scanning is completed.
More specifically, when the printhead has 512 nozzles and performs printing which is completed by four passes, the sheet feed amount after one scanning in the main scanning direction is almost equal to a printhead printing width of 512÷4=128 nozzles. At this time, the same raster on the sheet surface is always printed by different nozzles of the printhead in respective passes. In an example in which the printhead has 512 nozzles and performs 4-pass printing, a raster printed by the first nozzle counted from the uppermost end of the printhead in the first pass shifts by 128 nozzles in the second pass, and is identical to a raster printed by the 129th nozzle counted from the uppermost end of the printhead. From this principle, when the first nozzle counted from the uppermost end of the printhead is a discharge failure nozzle, data to be printed by the first nozzle is printed by the 129th nozzle counted from the uppermost end of the printhead in the second pass. In this manner, printing can be achieved by complementing the discharge failure of the first nozzle.
Also in single-pass printing, a discharge failure can be complemented in principle by setting a discharge failure complement printing pass in addition to a normal printing pass.
When the first nozzle counted from the uppermost end of the printhead is a discharge failure nozzle in a printhead having 512 nozzles, similar to the above-described example, single-pass printing is normally executed in the first pass. The sheet is then conveyed by a printhead printing width of 128 nozzles, and the 129th nozzle counted from the upper end of the printhead prints printing data which should be used by the first nozzle serving as a discharge failure nozzle. At this time, no printing is done by another nozzle, thereby implementing complement of a discharge failure.
There is also known an arrangement in which nozzles other than a discharge failure nozzle print in scanning a carriage in the forward direction, then the sheet is slightly conveyed, and another nozzle prints in an area not printed due to a discharge failure in scanning the carriage in the backward direction (see, e.g., Japanese Patent Publication Laid Open No. 8-25700).
To complement a discharge failure by the conventional method, the carriage must be scanned at least twice in the scanning direction.
As another discharge failure complement method, for example, Japanese Patent Publication Laid Open No. 2002-19101 discloses a method of performing complement in the same scanning using a nozzle used for discharging another color ink, and a method of increasing the printing duty of a nozzle adjacent to a discharge failure nozzle thereby complementing a part which is not printed owing to a discharge failure.
However, such a conventional discharge failure complement technique poses the following problems and is rarely employed.
Multi-pass printing will be considered first.
One of printing methods often adopted in current printers is “margin-less printing”. In this printing mode, when the printing paper size is, e.g., A4 size, printing is done on the entire sheet surface of this size. Generally in this printing at portions (in the sub-scanning direction) corresponding to the upper and lower margins of a paper sheet, the sheet conveyance amount changes even in the use of the same multi-pass printing method. For example, in 4-pass printing using a printhead having 512 nozzles, the sheet conveyance amount is almost equal to a printhead printing width of 128 nozzles, as described above. At portions corresponding to the upper and lower marginal areas of a paper sheet, printing is done using not all the 512 nozzles, but only some nozzles, e.g., 128 nozzles. At this time, the sheet conveyance amount is 128÷4=32 nozzles.
In this case, a raster printed by the first nozzle counted from the upper end of the printhead shifts by 32 nozzles in the second pass, and is identical to a raster printed by the 33rd nozzle counted from the upper end of the printhead. This means that the position of a target nozzle to be complemented dynamically changes on the same printing sheet surface, unlike a case where, when the first nozzle counted from the uppermost end of the printhead is a discharge failure nozzle, similar to the above-described example, it is unconditionally decided that printing data corresponding to the discharge failure nozzle can be complemented by the 129th nozzle counted from the uppermost end of the printhead.
It is a heavy burden on a conventional printing system to process in real time to a certain degree the relationship between a discharge failure nozzle and a complementary nozzle that dynamically changes. In the use of a printhead in which 512 nozzles are arranged in six arrays in the scanning direction of the printhead in order to cope with printing using inks of six different colors, a discharge failure cannot be complemented in practice if discharge failure nozzles occur at different positions of the nozzle arrays.
Discharge failure complement in the above-mentioned single-pass printing requires redundant scanning in the main scanning direction for only the complement process, which decreases the printing speed.
As a method of solving this problem, there is proposed a method of completing discharge failure complement by only one scanning of the printhead without utilizing discharge failure complement by multi-pass printing (see, e.g., Japanese Patent Publication Laid Open No. 2002-19101).
According to this method, when a discharge failure nozzle exists in the printhead, printing data assigned to this nozzle is distributed to a normal printing nozzle of the same nozzle array that exists near the discharge failure nozzle. This method can eliminate the need for a complicated data process over multiple passes even in discharge failure complement. No printing pass for only discharge failure complement exists, and a high-speed process can be easily achieved at a relatively low cost.
However, the conventional technique of completing discharge failure complement by only one scanning of the printhead suffers the following problems.
That is, the method of distributing printing data assigned to a discharge failure nozzle to a normal printing nozzle of the same nozzle array that exists near the discharge failure nozzle cannot avoid in principle a phenomenon in which a dot is printed at a position slightly deviated from an originally assigned position on the sheet surface. Especially in an image with a gradation of light colors, a white stripe might appear at a position (i.e., a discharge failure-complemented raster) where the printing raster of the discharge failure nozzle should exist.
Assume that the printhead has 512 nozzles and these nozzles are regularly arranged at an interval of 1,200 dpi. In this case, the presence of one discharge failure nozzle in the nozzle array means that one of print dots which should be regularly arranged at an interval of 1,200 dpi in the printed image is not printed. As a result, a white stripe is formed at a width of 1,200 dpi in the printed image. When the above-described conventional method of completing discharge failure complement by only one scanning of the printhead is applied, even complementary printing may not completely complement the 1,200-dpi wide white stripe by moving one print dot assigned to the discharge failure nozzle to a normal nozzle of the same nozzle array that exists near the discharge failure nozzle.
In multi-pass printing (e.g., 4-pass printing or 8-pass printing) completed by many passes, the number of dots printed by one scanning of the printhead is small. For a smaller number of dots printed by one scanning, the number of print dots assigned to a discharge failure nozzle becomes smaller, and the complement count in one raster also becomes smaller. This makes the 1,200-dpi wide white stripe less conspicuous. To the contrary, in 2-pass printing or single-pass printing, the number of dots printed by one scanning is relatively large, and the white stripe still stands out.
As described above, the image quality inevitably degrades in the conventional method of completing discharge failure complement by one scanning of the printhead.